---
title: "H3 Grid Support"
author: "Gilles Colling"
date: "`r Sys.Date()`"
output: rmarkdown::html_vignette
vignette: >
  %\VignetteIndexEntry{H3 Grid Support}
  %\VignetteEngine{knitr::rmarkdown}
  %\VignetteEncoding{UTF-8}
---

```{r setup, include = FALSE}
CAN_RUN <- requireNamespace("sf", quietly = TRUE)

knitr::opts_chunk$set(
  collapse = TRUE,
  comment = "#>",
  fig.width = 7,
  fig.height = 5,
  eval = CAN_RUN
)
library(hexify)
```

hexify v0.5.0 adds [H3](https://h3geo.org/) as a first-class grid type alongside
ISEA. Every core function --- `hexify()`, `grid_rect()`, `grid_clip()`,
`get_parent()`, `get_children()` --- works with both grid systems through the
same interface. This vignette covers what H3 is, how to use it in hexify,
and when to prefer it over ISEA.

## What is H3?

H3 is a hierarchical hexagonal grid system developed by Uber. It partitions
Earth's surface into hexagonal cells at 16 resolutions (0--15), each roughly
7$\times$ finer than the last. Cell IDs are 64-bit integers encoded as
hexadecimal strings (e.g., `"8528342bfffffff"`).

H3 has become an industry standard adopted by the FCC, Foursquare, and
numerous geospatial platforms.

**Key difference from ISEA**: H3 cells are _not_ equal-area. Cell area varies
by ~1.6$\times$ between the largest and smallest hexagons at any given
resolution, depending on latitude. For rigorous equal-area analysis, use ISEA.
For interoperability with H3 ecosystems, use `type = "h3"`.

## Getting Started

Create an H3 grid by passing `type = "h3"` to `hex_grid()`:

```{r h3-grid}
library(sf)
library(ggplot2)

# Create an H3 grid specification
grid_h3 <- hex_grid(resolution = 5, type = "h3")
grid_h3
```

Then use `hexify()` with the grid object, just like ISEA:

```{r h3-hexify}
# Sample cities
cities <- data.frame(
  name = c("Vienna", "Paris", "Madrid", "Berlin", "Rome",
           "London", "Prague", "Warsaw", "Budapest", "Amsterdam"),
  lon = c(16.37, 2.35, -3.70, 13.40, 12.50,
          -0.12, 14.42, 21.01, 19.04, 4.90),
  lat = c(48.21, 48.86, 40.42, 52.52, 41.90,
          51.51, 50.08, 52.23, 47.50, 52.37)
)

result <- hexify(cities, lon = "lon", lat = "lat", grid = grid_h3)
result
```

H3 cell IDs are character strings, unlike ISEA's numeric IDs:

```{r h3-cell-ids}
# Cell IDs are hexadecimal strings
result@cell_id

# All standard accessors work
cells(result)
n_cells(result)
```

### Choosing Resolution by Area

If you think in terms of cell area rather than resolution numbers, pass
`area_km2` instead of `resolution`. hexify picks the closest H3 resolution:

```{r h3-by-area}
grid_area <- hex_grid(area_km2 = 500, type = "h3")
grid_area
```

## Grid Generation

All grid generation functions work with H3 grids.

### Rectangular Region

```{r h3-grid-rect, fig.width=7, fig.height=5}
# Generate H3 hexagons over Western Europe
grid_h3 <- hex_grid(resolution = 3, type = "h3")
europe_h3 <- grid_rect(c(-10, 35, 25, 60), grid_h3)

# Basemap
europe <- hexify_world[hexify_world$continent == "Europe", ]

ggplot() +
  geom_sf(data = europe, fill = "gray95", color = "gray60") +
  geom_sf(data = europe_h3, fill = NA, color = "#E6550D", linewidth = 0.4) +
  coord_sf(xlim = c(-10, 25), ylim = c(35, 60)) +
  labs(title = sprintf("H3 Resolution %d Grid (~%.0f km² avg cells)",
                       grid_h3@resolution, grid_h3@area_km2)) +
  theme_minimal()
```

### Clipping to a Boundary

```{r h3-grid-clip, fig.width=6, fig.height=6}
# Clip H3 grid to France
france <- hexify_world[hexify_world$name == "France", ]
grid_h3 <- hex_grid(resolution = 4, type = "h3")
france_h3 <- grid_clip(france, grid_h3)

ggplot() +
  geom_sf(data = france, fill = "gray95", color = "gray40", linewidth = 0.5) +
  geom_sf(data = france_h3, fill = alpha("#E6550D", 0.3),
          color = "#E6550D", linewidth = 0.3) +
  coord_sf(xlim = c(-5, 10), ylim = c(41, 52)) +
  labs(title = sprintf("H3 Grid Clipped to France (res %d)", grid_h3@resolution)) +
  theme_minimal()
```

## Hierarchical Navigation

H3's killer feature is its clean hierarchical structure: every cell has exactly
one parent and seven children. hexify exposes this with `get_parent()` and
`get_children()`.

### Parents

```{r h3-parent}
# Get parent cells (one resolution coarser)
grid_h3 <- hex_grid(resolution = 5, type = "h3")
child_ids <- lonlat_to_cell(
  lon = c(16.37, 2.35, 13.40),
  lat = c(48.21, 48.86, 52.52),
  grid = grid_h3
)

parent_ids <- get_parent(child_ids, grid_h3, levels = 1)
data.frame(child = child_ids, parent = parent_ids)
```

### Children

```{r h3-children}
# Get children of a single cell (one resolution finer)
grid_coarse <- hex_grid(resolution = 3, type = "h3")
coarse_id <- lonlat_to_cell(16.37, 48.21, grid_coarse)

children <- get_children(coarse_id, grid_coarse, levels = 1)
cat(length(children[[1]]), "children at resolution", grid_coarse@resolution + 1, "\n")
head(children[[1]])
```

### Visualizing the Hierarchy

```{r h3-hierarchy-plot, fig.width=7, fig.height=5}
# Parent cell polygon
parent_poly <- cell_to_sf(coarse_id, grid_coarse)

# Children cell polygons
grid_fine <- hex_grid(resolution = 4, type = "h3")
children_poly <- cell_to_sf(children[[1]], grid_fine)

ggplot() +
  geom_sf(data = children_poly, fill = alpha("#E6550D", 0.3),
          color = "#E6550D", linewidth = 0.5) +
  geom_sf(data = parent_poly, fill = NA, color = "black", linewidth = 1.2) +
  labs(title = sprintf("H3 Hierarchy: 1 parent (res %d) → %d children (res %d)",
                       grid_coarse@resolution,
                       length(children[[1]]),
                       grid_fine@resolution)) +
  theme_minimal()
```

## Working with H3 Data

The standard hexify workflow applies to H3 grids. Here's a complete example
using simulated species observations:

```{r h3-workflow, fig.width=8, fig.height=6, message=FALSE, warning=FALSE}
set.seed(42)

# Simulate observations across Europe
obs <- data.frame(
  lon = c(rnorm(200, 10, 12), rnorm(100, 25, 8)),
  lat = c(rnorm(200, 48, 6), rnorm(100, 55, 4)),
  species = sample(c("Sp. A", "Sp. B", "Sp. C"), 300, replace = TRUE)
)
obs$lon <- pmax(-10, pmin(40, obs$lon))
obs$lat <- pmax(35, pmin(65, obs$lat))

# Hexify with H3
grid_h3 <- hex_grid(resolution = 3, type = "h3")
obs_hex <- hexify(obs, lon = "lon", lat = "lat", grid = grid_h3)

# Aggregate: species richness per cell
obs_df <- as.data.frame(obs_hex)
obs_df$cell_id <- obs_hex@cell_id

richness <- aggregate(species ~ cell_id, data = obs_df,
                      FUN = function(x) length(unique(x)))
names(richness)[2] <- "n_species"

# Map it
polys <- cell_to_sf(richness$cell_id, grid_h3)
polys <- merge(polys, richness, by = "cell_id")

europe <- hexify_world[hexify_world$continent == "Europe", ]

ggplot() +
  geom_sf(data = europe, fill = "gray95", color = "gray70", linewidth = 0.2) +
  geom_sf(data = polys, aes(fill = n_species), color = "white", linewidth = 0.3) +
  scale_fill_viridis_c(option = "plasma", name = "Species\nRichness") +
  coord_sf(xlim = c(-10, 40), ylim = c(35, 65)) +
  labs(title = "Species Richness on H3 Grid",
       subtitle = sprintf("H3 resolution %d (~%.0f km² avg cells)",
                          grid_h3@resolution, grid_h3@area_km2)) +
  theme_minimal() +
  theme(axis.text = element_blank(), axis.ticks = element_blank())
```

## ISEA--H3 Crosswalk

hexify v0.6.0 added `h3_crosswalk()` for bidirectional mapping between ISEA
and H3 cell IDs. This is useful when you work in ISEA for analysis but need to
share results with H3 ecosystems (or vice versa).

```{r h3-crosswalk}
# Start with an ISEA grid and some cells
grid_isea <- hex_grid(resolution = 9, aperture = 3)
isea_ids <- lonlat_to_cell(
  lon = c(16.37, 2.35, 13.40, -3.70, 12.50),
  lat = c(48.21, 48.86, 52.52, 40.42, 41.90),
  grid = grid_isea
)

# Map ISEA cells to their closest H3 equivalents
xw <- h3_crosswalk(isea_ids, grid_isea)
xw[, c("isea_cell_id", "h3_cell_id", "isea_area_km2", "h3_area_km2")]
```

The `area_ratio` column shows how ISEA and H3 cell sizes compare ---
values close to 1 mean the resolutions are well-matched.

## ISEA vs H3: When to Use Which

| | ISEA | H3 |
|---|---|---|
| **Cell area** | Exactly equal | ~1.6$\times$ variation |
| **Cell IDs** | Numeric (integer) | Character (hex string) |
| **Apertures** | 3, 4, 7, 4/3 | Fixed (7) |
| **Resolutions** | 0--30 | 0--15 |
| **Hierarchy** | Approximate (aperture-dependent) | Exact (7 children per parent) |
| **Dependencies** | None (built-in C++) | None (vendored H3 C library) |
| **Industry adoption** | Scientific / government | Tech industry / commercial |

**Use ISEA when**:

- Equal-area cells are required (biodiversity surveys, density estimation,
  statistical sampling)

- You need fine control over aperture and resolution

- No external dependencies are acceptable

**Use H3 when**:

- Interoperability with H3 ecosystems (Uber, Foursquare, DuckDB, BigQuery)

- Clean hierarchical operations (parent/child traversal) are a priority

- Slight area variation across latitudes is acceptable for your analysis

## Resolution Reference

```{r h3-resolution-table}
h3_res <- hexify_compare_resolutions(type = "h3", res_range = 0:15)
h3_res$n_cells_fmt <- ifelse(
  h3_res$n_cells > 1e9,
  sprintf("%.1fB", h3_res$n_cells / 1e9),
  ifelse(h3_res$n_cells > 1e6,
         sprintf("%.1fM", h3_res$n_cells / 1e6),
         ifelse(h3_res$n_cells > 1e3,
                sprintf("%.1fK", h3_res$n_cells / 1e3),
                as.character(h3_res$n_cells)))
)
knitr::kable(
  h3_res[, c("resolution", "n_cells_fmt", "cell_area_km2", "cell_spacing_km")],
  col.names = c("Resolution", "# Cells", "Avg Area (km²)", "Spacing (km)"),
  digits = 1
)
```

Areas are averages --- actual cell area varies by latitude.

## See Also

- `vignette("quickstart")` - Getting started with hexify (ISEA-focused)

- `vignette("workflows")` - Grid generation, multi-resolution analysis, GIS export

- `vignette("visualization")` - Plotting with `plot()`, `hexify_heatmap()`

- `vignette("theory")` - Mathematical foundations of the ISEA projection
